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The transistor ranks as one of the greatest inventions of
20th century technology. It finds application in virtually all
electronic devices from radios to computers. Integrated circuits
typically contain millions of transistors, formed on a single tiny
chip of silicon. Two of the basic uses of a transistor, which will be
explored in this experiment, are as an amplifier and as a switch.

The php transistor, shown in Fig. 1a contains three distinct regions,
a p-type ”emitter”, an n-type “base” and a p-type “collector”, which
together form two pn junctions. In a typical amplifier circuit,
voltages are supplied so that the emitter-base junction is
forward-biased and the collector-base junction is reverse-biased. This
means that . Fig. 1c illustrates a “common emitter” circuit,
so called because the emitter is common to the input circuit on the
left and the output circuit on the right.

Consider first the forward-biased emitter-base junction. The doping
of the emitter is made much heavier than that of the base so that
positive holes from the emitter form almost all of the current, ,
from emitter to base. The base, being lightly doped, does not have
many electrons available for recombination with these holes to form
neutral atoms. It is also very narrow () making it easy for a
large fraction, , of the holes to diffuse across to the
collector-base junction where the junction voltage accelerates them
into the collector region to form the collector current, .

Thus,

(1)

The remaining fraction, , of holes leave the base
through the external connection to form the base current, ,
where

(2)

The “current gain”, , of the transistor is defined by

(3)

For typical transistors, , giving
values of . Thus we have a “current
amplifier”, in that a small change in will cause a large change in
. The “voltage gain”, , is the ratio of the
voltage drop, ,
across the output resistor, , to the voltage, , of the input
source:

(4)

Applying the loop theorem to the input circuit in Fig 1c,
and assuming , one obtains

(5)

where is the resistance of the emitter-base junction.
By a suitable choice of resistors, an appreciable
voltage amplification may be obtained.

Figure 2: The AC amplifier circuit.

Fig. 2 shows how the transistor may be used as an AC amplifier to
amplify a small signal from a signal generator (e.g. the output of the
DAC, a digital to analog converter, of a CD or MP3 player). The two
batteries in the circuit behave like large capacitors with impedances
. Once again the voltage gain is given by equation
(5). However, this is a simplified situation. In reality, the
transistor junctions possess
capacitance, and the corresponding reactances are frequency
dependent. Thus we can expect Av to be a function of frequency.

Set up the circuit shown in Fig. 1c using the power supply outputs for
the voltages and . Note the symbols
e, b and c denoting the transistor connections. Use a
resistor for and a
resistor for . Turn the supply outputs to zero then turn on
the unit. Set one of the digital meters to the 20 V-DC range and
connect it to measure (+ lead to ground on the
transistor board). Adjust to approximately 15
V. Reconnect the meter to measure . This should also
read 15 V, indicating . Connect the second meter to measure
(also with the + lead to ground). Slowly increase up to 2 V and
note decreasing, indicating an increasing . Your amplifier is
now working.

Now reconnect the meter
to measure . (The first meter should still be measuring .)
Adjust to 1, 3, 5, .... 15 V and calculate
corresponding values of

(7)

Repeat with = 0.90 and 1.05 V. Plot on a single graph
(y-axis) vs ; (x-axis) for each value of
. The “operating region” of the transistor is where the
curves level off. Determine for the middle curve at
= 3 V. Is
generally constant?

Reconnect the meters to measure and , set
to 6 V and decrease to zero. Replace
with the light bulb. Slowly increase until the bulb is at maximum
intensity. Connect the two-way switch as shown in Fig. 3. Note the
effect of operating the switch. Measure to determine the very small
current that you are turning on and off to control the much larger
current (~ 1A) through the light bulb. ,

Reconnect the circuit of Fig 1c with the meters to measure
and . Increase to 15 V and
turn down to zero. Increase until
= 7.5 V. Connect the signal generator in series with
as shown in Fig. 2 and adjust it to 100Hz. Connect the
oscilloscope also as in Fig. 2WITH THE BLACK LEADS TO GROUND ON THE
TRANSISTOR BOARD FOR BOTH CONNECTIONS. Observe the input and output
waveforms. Note that adjusting causes distortion of the output
waveform. Can you explain this? From the ratio of peak-to-peak
voltages, determine for frequencies of 100 Hz, 1 kHz, 10 kHz and
20kHz. Would this amplifier be good as an audio-amplifier? Replace
with a resistor and note the effect on . Is this an expected
result?